In the processing of semiconductor wafers by plasma etching, chemical vapor deposition, photolithographic etching and other means there is a need to precisely locate and align the wafer. Traditional means used mechanical pins that, through contact with the edge of the wafer; were able to determine the edge and then calculate the wafer's center. The use of pins is not sufficiently accurate for some microelectronic circuitry work. Additionally, the contact from pins has the effect of producing particulates which interfere with processing of the wafer.
A non-contact technique of determining the edge and center of a wafer involves the use of a movable detector including a light source and light detector which are spaced apart to allow a wafer to pass therebetween. Other non-contact techniques include height sensors for determining the location of a wafer flat (see U.S. Pat. No. 4,328,553) or an array of sensors which are located along a path of movement of the wafer (see U.S. Pat. No. 4,819,167). Although these techniques avoid contact of the wafer with pins and thus avoid the problem of generating particulates, they have limitations on accuracy. Additional inaccuracies in the image could result from movement of the light detector or wafer during the measurement process. Such inaccurate position readings could thus produce inaccurate results in the determination of the center and orientation of the semiconductor wafer. Given the increasing need to reduce the size of components in chip manufacture there is an ever increasing need to be able to precisely determine both the center and orientation of semiconductor wafers during processing.
Currently available methods for determining a wafer's center require the use of moving parts to scan, rotate or move contact pins. This increases the cost of the wafer centering device and increases the chances of mechanical failure. It is an object of the present invention to determine whether a wafer is properly centered and orientated without the need for moving parts.
A system for positioning of semiconductor wafers is disclosed in commonly owned U.S. Pat. No. 4,833,790. As shown in FIG. 1, the positioning system includes a wafer shuttle 12, a position sensor 14, a rotatable spindle 16 on a base 17 and a central controller 24 comprising a programmable digital computer. The wafer shuttle 12 retrieves wafers W from a wafer cassette 18 and transports the wafers to the spindle 16 where the wafers are centered and aligned in a desired orientation prior to being removed by an articulated wafer transport arm 20 having a first segment 20a and a cradle segment 20b, both of which are located in an air lock 22 associated with processing equipment such as a plasma etch system, CVD reactor, or the like.
The wafer cassette 18 includes horizontal shelves 19 supporting individual wafers and the cassette is movable vertically by an elevator platform. The wafer shuttle 12 includes a carriage 40 mounted to horizontally reciprocate on a pair of guide rails 42 with the position of the carriage 40 being controlled by an electric drive motor 44 controlled by controller 24 through communication line 46 which passes into interface 26. A I-shaped support arm 50 secured to carriage 40 reciprocates along a linear path between the spindle 16 and the wafer cassette 18 as the carriage 40 is driven back and forth along guide rails 42. The support arm 50 can include vacuum ports for securing the wafer during transport. The arm 50 can be retracted to place a wafer over spindle 16 and spindle 16 can be raised so that the wafer is lifted above the arm 50.
The spindle 16 can include a vacuum port for firmly securing the wafer thereto. When placed on the spindle 16, the center of the wafer will be offset from the center of the spindle by an unknown distance in an unknown direction. The position sensor 14 includes a carriage 60 mounted on a rotating drive screw 62 driven by motor 64 which in turn is supervised by controller 24 through communication line 66. The carriage 60 includes an optical detector 68 and by translating the carriage 60 back and forth, the location of the periphery of the wafer along the linear path between the spindle and the cassette can be determined. The optical sensor 68 can be a light emitting diode source and a phototransistor detector.
The centering operation is described with reference to FIG. 2. In operation, the distance r.sub.1, between the center of rotation CR and a point P.sub.1, on the periphery of the wafer W, is measured after rotating the wafer through an angle .THETA..sub.1 from an arbitrary baseline BL drawn through the center of rotation. The values of the radius r.sub.1 and angle .THETA..sub.1, are then stored in the controller 24. The wafer is then further rotated through a second angle .theta..sub.2 relative to the baseline BL and the distance r.sub.2 between the center of rotation CR and a point P.sub.2 on the periphery of the wafer W is measured. Similar measurements are then made for a third point P.sub.3 and when the measurements are completed, the length of offset l and angular offset .alpha. are calculated according to mathematical formulas. Once the offset angle .alpha. and offset length l have been determined, the wafer can be rotated by the spindle 16 so that the line between the center of rotation CR and the center of the wafer CW is aligned with the direction of the linear path travelled by the support arm 50. The wafer is then lowered onto the support arm 50 by retracting spindle 16 after which the support arm 50 is translated in the direction necessary to align the center of the wafer CW with the center of rotation CR. The spindle 16 can then be raised and the wafer is ready for further manipulation and processing.
It is an object of the present invention to provide a method and apparatus to precisely locate the center and orientation of a semiconductor wafer in a manner which overcomes limitations of the prior art.